More stringent fuel economy regulations have prompted the need for improved efficiency of the internal combustion (IC) engine. Lightweighting, friction reduction, thermal management, variable valve timing and a diverse array of variable valve lift technologies are all part of the technology toolbox for IC engine designers.
Variable valve lift (VVL) systems typically employ a technology in a valve train of an IC engine that allows different engine valve lifts to occur. The valve train consists of the components that are required to actuate an engine valve, including a camshaft, the valve and all components that lie in between. VVL systems are typically divided into two categories: continuous variable and discrete variable. Continuous variable valve lift systems are capable of varying a valve lift from a design lift minimum to a design lift maximum to achieve any of several lift heights. Discrete variable valve lift systems are capable of switching between two or three distinct valve lifts. Components that enable these different valve lift modes are often called switchable valve train components. Typical two-step discrete valve lift systems switch between a full valve lift mode and a partial valve lift mode, often termed cam profile switching, or between a full valve lift mode and a no valve lift mode that facilitates deactivation of the valve. Valve deactivation can be applied in different ways. In the case of a four-valve-per-cylinder configuration (two intake+two exhaust), one of two intake valves can be deactivated. Deactivating only one of the two intake valves can provide for an increased swirl condition that enhances combustion of the air-fuel mixture. In another scenario, all of the intake and exhaust valves are deactivated for a selected cylinder which facilitates cylinder deactivation. On most engines, cylinder deactivation is applied to a fixed set of cylinders, when lightly loaded at steady-state speeds, to achieve the fuel economy of a smaller displacement engine. A lightly loaded engine running with a reduced amount of active cylinders requires a higher intake manifold pressure, and, thus, greater throttle plate opening, than an engine running with all of its cylinders in the active state. Given the lower intake restriction, throttling losses are reduced in the cylinder deactivation mode and the engine runs with greater efficiency. For those engines that deactivate half of the cylinders, it is typical in the engine industry to deactivate every other cylinder in the firing order to ensure smoothness of engine operation while in this mode. Deactivation also includes shutting off the fuel to the dormant cylinders. Reactivation of dormant cylinders occurs when the driver demands more power for acceleration. The smooth transition between normal and partial engine operation is achieved by controlling ignition timing, cam timing and throttle position, as managed by the engine control unit (ECU). Examples of switchable valve train components that serve as cylinder deactivation facilitators include roller lifters, pivot elements, rocker arms and camshafts; each of these components is able to switch from a full valve lift mode to a no valve lift mode. The switching of lifts occurs on the base circle or non-lift portion of the camshaft; therefore the time to switch from one mode to another is limited by the time that the camshaft is rotating through its base circle portion; more time for switching is available at lower engine speeds and less time is available at higher engine speeds. Maximum switching engine speeds are defined by whether there is enough time available on the base circle portion to fully actuate a locking mechanism to achieve the desired lift mode.
The precision of control of the deactivated cylinders varies within the engine industry. For optimum performance of the system, selective cylinder control rather than simultaneous multiple cylinder control is recommended. With selective cylinder control, the timing of the valve deactivation event with respect to the combustion cycle is maintained for each individual cylinder; for example, in a selective cylinder control system, an exhaust charge is normally trapped in the cylinder, which serves as an air spring and aids oil control during the deactivated mode. This is typically accomplished by deactivating the exhaust valve(s) first, followed by deactivation of the intake valve(s) of a given cylinder. With simultaneous multiple cylinder control, the timing of the valve deactivation event with respect to the combustion cycle is not controlled to the extent of the selective cylinder control resulting in intermittent exhaust charge trapping.
In today's IC engines, many of the switchable valve train components that enable valve deactivation for cylinder deactivation contain a locking mechanism that is actuated by an electro-hydraulic system. The electro-hydraulic system typically contains at least one solenoid valve within an array of oil galleries that manages engine oil pressure to either lock or unlock the locking mechanism within the switchable valve train component to enable a valve lift switching event. These types of electro-hydraulic systems require time within the combustion cycle to actuate the switchable valve train component. FIG. 1 shows a diagram of a total actuation time, termed “system response time” for a typical electro-hydraulic system to unlock a switchable valve train component. The system response time can be broken into three segments. A first time segment, solenoid valve response time (termed “magnetic”), is defined as a time required for a coil of a solenoid valve to build enough current to move an armature through its stroke after the solenoid receives its signal from an ECU. A second time segment, hydraulic pressure propagation (termed “hydraulic”), is defined as a time required for oil pressure to propagate within an oil control gallery from a solenoid valve to a locking mechanism of a switchable valve train component. A third time segment, locking mechanism travel (termed “mechanical”), is defined as a time required for the locking mechanism to complete its actuation stroke. The first time segment is influenced by system voltage, oil pressure, oil temperature, tolerances/fits of components of the solenoid valve, and contamination; the second time segment is influenced by oil gallery volume, oil circuit flow restrictions, oil aeration, oil temperature and oil pressure; and the third time segment is influenced by oil aeration, oil temperature, oil pressure, tolerances/fits of the locking mechanism, displaced volume of the locking mechanism(s), and contamination. One that is familiar with the art of hydraulic switching valve train components can understand that the second time segment (hydraulic) is significantly dependent on the kinematic viscosity of the oil, which is a function of oil grade and temperature. For this reason, the allowable deactivated mode operating range is often limited by oil temperature due to increased system response times with reduced oil temperatures.
In most IC engine applications, switchable valve train components for cylinder deactivation in an electro-hydraulic system are classified as “pressureless-locked”, which equates to:                a). In a no or low oil pressure condition, the spring-biased locking mechanism will be in a locked position, facilitating the function of a standard valve train component that translates rotary camshaft motion to linear valve motion; and,        b). In a condition in which engine oil pressure is delivered to the locking mechanism that exceeds the force of the locking mechanism bias spring, the locking mechanism will be displaced a given stroke to an unlocked position, facilitating valve deactivation where the rotary camshaft motion is not translated to the valve.        
“Pressureless-unlocked” electro-hydraulic systems can be found in some cam profile switching systems that switch between a full valve lift and a partial valve lift, which equates to:                a). In a no or low oil pressure condition, the spring-biased locking mechanism will be in an unlocked position, facilitating a partial valve lift event; and,        b). In a condition in which engine oil pressure is delivered to the locking mechanism that exceeds the force of the locking mechanism bias spring, the locking mechanism will be displaced a given stroke to a locked position, facilitating a full valve lift event.        
Switching valve train components are often larger in size and mass when compared to non-switching or standard valve train components and remains a constant challenge. The increased mass typically equates to an increased rotational mass or mass moment of inertia, which requires a potential increase in valve spring force to maintain contact of the valve train components throughout the entire engine speed range. Such a force increase often equates to increased stresses, wear, and friction between rubbing interfaces of the valve train system. For this reason, a minimized rotational mass is always desired in a switching valve train component.
With the successful implementation of cylinder deactivation systems on millions of production engines, engine manufacturers are now looking to expand the operating range. Examples include switching at higher engine speeds along with switching at colder oil temperatures. In addition, a new type of cylinder deactivation is in development that expands the deactivated mode operating range, increases the number of deactivating cylinders, and increases the frequency of switching in and out of a deactivated mode. In this new type of cylinder deactivation, all cylinders, as opposed to a group of cylinders, are continuously switched on and off depending on the demanded engine output. By controlling the engine output over a larger operating range in this way instead of by conventional throttling, pumping losses are reduced even further compared to traditional cylinder deactivation systems and, thus, a higher engine efficiency is achieved. In order to meet the increased system demands of such new cylinder deactivation systems, a solution to reduce the actuation time for valve deactivation and subsequent reactivation is needed. Furthermore, a solution is required that does not add additional rotating mass to the switching valve train component in order to minimize the valve spring force requirements, which reduces stress, wear, and friction in the valve train system.